Please refer to FIG. 1(a). FIG. 1(a) is a schematic view showing the supporting base 10 and the guiding shaft 11 cooperating with each other inside an image scanner according to the prior art. It shows clearly in the view that the supporting base 10 is mounted on the guiding shaft 11 for being slid along the guiding shaft 11.
Please refer to FIG. 1(b). FIG. 1(b) is a sectional view showing the structures of the shaft sleeve 101 and the bearing 102 which are needed when the supporting base is mounting on the guiding shaft 11 according to the prior art. The shaft sleeve 101 is manufactured together with the housing of the supporting base 10. The bearing 102, which is usually made of metal, is mounted at two ends of the shaft sleeve 101 by way of being pressed thereinto directly after the shaft sleeve 101 is formed. The relationship between the. central parallel precision of two end bearings 102 and the clearance between the bearing 102 and the guiding shaft 11 could be: (1) the central parallel precision of two end bearings 102 has a swing value smaller than the clearance between the bearing 102 and the guiding shaft 11. Therefore, by reducing the clearance between the bearing 102 and the guiding shaft 11, a smallest clearance and a smallest swing value in an optic module can be obtained and the best image quality is obtained then. (2) The central parallel precision of two end bearings 102 has a swing value larger than the clearance between the bearing 102 and the guiding shaft 11. Therefore, when the clearance between the bearing 102 and the guiding shaft 11 is not large enough, it might cause the dryness and jamming and result in the scanning operation stopped. In such way, the clearance between the bearing 102 and the guiding shaft 11 must be increased in order to obtain a largest clearance and a largest swing value in an optic module. However, the image quality is thus usually not good.
Therefore, the swing value of the central parallel precision of two bearings 102 is related to the image quality. The central precision at two ends of the shaft sleeve 101 on the optic module main body is related to the integral precision after the bearing 102 is assembled. It is therefore the main problem how we can obtain a perfect track. In the present time, the shaft sleeve design of the optic module main body must consider the complexity of the module structure, the easiness of drawing the mold or the mold injection and the structural strength of end product. Generally, it can be divided into two forms as follows:
The first kind of shaft sleeve structure as shown in FIG. 2(a) has a structural feature that the central part 201 of the shaft sleeve 20 is cut-out. During the process of manufacturing the shaft sleeve 20, the shaft sleeve 20 is obtained by shaping the forward slider and the backward slider (not shown in the drawing) respectively. The sliders have shorter length so that the two sliders can be driven to be drawn out along the guiding member (not shown in the drawing) by the driving force while drawing the mold. The advantage is that mold injection is easy to be accomplished and the concentric precision can still be maintained in the beginning without any extra auxiliary mechanism. Nevertheless, the disadvantage is that the slider contacts with the guiding member (not shown in the drawing) directly so that the abrasion occurs with the production time and the production quantity, and the concentric precision starts to be reduced. In addition, due to the central part is cut-out, the structural strength of the end product is weaker.
The second kind of shaft sleeve structure as shown in FIG. 2(b) has a structural feature that the central part 211 of the shaft sleeve 21 is intact. During the process of manufacturing the shaft sleeve 21, the shaft sleeve 21 is directly shaped from one single slider with an identical diameter. Therefore, the advantage is that the concentric precision is higher, the structural strength of the product is stronger, and the mold structure is simpler. However, since the slider is longer, the disadvantage is that an auxiliary mechanism (oil pressure cylinder) is needed to drive the slider to be drawn out. In addition, due to the diameter of the slider is identical, the mold-drawing angle is so small that it is almost zero. The mold injection is thus more difficult. Besides, the internal diameters of the forward shaft sleeve and the backward shaft sleeve are often different, so the precision can not be controlled easily after assembled.
From the above, the different shaping ways of shaft sleeve and the different structures thereof will affect the scanning quality and the enduring ability of an image scanner seriously. Thus, how to rectify the foresaid conventional drawback is the main purpose of the present invention.